In the North American telephone system it is common practice to allow two or more telephone sets to operate in parallel, as extensions, simultaneously on the same analogue telephone line. When analogue voice transmission is used on the telephone line (i.e. subscriber loop), the signals from the two or more extensions (i.e. telephone sets) are added linearly simply due to the fact that the two or more extensions are connected in parallel across the same telephone line.
When digital signals are employed in the subscriber loop, the installation of extension telephone sets is not as simple as it was with analogue signals. In digital telephony, the analogue (e.g. voice) signals are encoded in a non-linear fashion such as in the nu-law pulse code modulation (PCM). Consequently it is not possible to add the digital signals directly, as such practice results in distortion of the signals.
In the prior art, when digital transmission was employed and when two or more digital signals were to be combined, as in a conference circuit, the digital signals were routed to a common circuit (e.g. a conference circuit or conference bridge) usually provided as common equipment in a central office.
There are many such conference circuits described in the art. Three of these prior circuits are briefly described below.
U.S. Pat. No. 4,160,878 dated July 10, 1979 to P. Hirschmann and E. Hoefer describes a circuit in which digital signals of all subscribers are consecutively converted into analog signals and an analog summation signal is formed therefrom. This summation signal is again converted back into a digital signal (from the abstract of the patent).
U.S. Pat. No. 4,007,338 dated Feb. 8, 1977 to D. W. McLaughlin describes a purely digital conferencing circuit that is based upon comparing the magnitude of the PCM digital samples. As stated at column 2, lines 30 to 44 of the patent "the operation is such that two words [for a three party conference] corresponding to two channels are read from the information memory every time slot, and after proper comparison, the largest sample is transmitted to the third channel. In other words, assuming, for example, that channels 3, 5 and 9 are engaged in a 3-way conference, during the channel 3 time slot, the samples from channel 5 and channel 9 are read and compared, and the larger of the two samples is transmitted to channel 3. Subsequently, during the channel 5 time slot, the samples from channel 3 and channel 9 are read and compared, and the larger of the two samples is transmitted. During the channel 9 time slot, the operation is repeated, with the largest of the channel 3 and 5 samples being transmitted."
U.S. Pat. No. 3,984,643 dated Oct. 5, 1976 to S. A. Inrig and A. S. J. Chapman describes a conferencing system in which, for an n-party conference, the PCM words for each of the n channels involved are linearized and then summed. The PCM word (linearized) from a particular conferee is then subtracted from the sum, and that result (i.e. the sum of n-1 conferees) is then converted back to PCM and sent to that particular conferee (see column 4, lines 7 to 30 of the patent).
The above three patents are similar in that the conferencing function in all three is provided by routing all the digital (PCM) signals to common equipment at one common location (e.g. at a telephone switching office). This need for routing all the PCM signals to common equipment at a common location can present problems when it is necessary to provide extension service capability for a large number of subscribers.